Combustible gas detector



Oct. 23, 1956 N. J. THOMPSON 2,768,069

COMBUSTIBLE GAS DETECTOR Filed June 5, 1952 United States PatentCOMBUSTIBLE GAS DETECTOR Norman J. Thompson, Needham, Mass., assignor toFactory Mutual Research Corporation, Boston, Mass., a corporation ofMassachusetts Application June 5, 1952, Serial No. 292,000 1 Claim. (Cl.23-255) The present invention relates to a thermally responsive devicefor detecting the presence of flammable gases, particularly to a heatedcatalytic device having a resistor element which varies in resistanceaccording to temperature changes produced by combustion of the flammablecomponents of gas.

It has been proposed to connect a coil of resistor wire in a resistancemeasuring circuit such as a Wheatstone bridge, heat the coil to thetemperature required for catalytically induced combustion of the gas tobe detected, and expose the resistor wire to a current of air containingthe flammable gas. Variation in the amount of gas in the air currentwill change the heat of combustion and produce a corresponding variationin the resistance of the wire. Variations in the wire resistanceindicate the amount of flammable gas present. While previous thermallyresponsive devices have operated satisfactorily for limited periods,they have proved to be inefficient and subject to rapid deterioration.If high current is applied to the relatively fragile resistor itconsumes power excessively when operated at a temperature high enough toinsure sensitivity and has a relatively short useful life, while ifoperated at low temperature it is insufiiciently sensitive to detect lowconcentrations of certain flammable gases and is more susceptible tocatalytic poisoning. Devices of the character referred to are alsouseful as fire detectors because the smoke and other products ofcombustion resulting from a fire contain unoxidized gas.

One object of the present invention is to afford a way of increasing thesensitivity of the thermally responsive device while reducing itsconsumption of power. Another object is to provide a simple,economically manufactured device which is physically more durable andcatalytically less susceptible to deterioration.

In one aspect the invention relates to a temperature responsive devicefor use in apparatus for detecting flammable gas, the device comprisingan electrical heating element for raising the gas to combustiontemperature, a separate thermo-sensitive element which changes value inresponse to combustion of flammable gas, means for electricallyinsulating the aforesaid elements from each other and holding them inspaced relation, and terminals associated with the respective elementsfor connecting them in different circuits so that the thermo-sensitiveelement is electrically as well as physically separated from the heatingresistance, thus allowing the heating element to be operated at optimumpower or current, while the sensitive element may be operated at highvoltage for maximum sensitivity but at relatively low current andtemperature to reduce power consumption and deterioration.

Preferably the aforesaid insulating means includes a hollow support, oneof the elements being mounted outside and one inside the support. Theheating element may be a relatively thick durable wire mounted outsidethe support, While the thermo-sensitive element may be a relatively finewire enclosed within and protected by the support. in one embodiment theinsulating means comprises a multi-threaded tubular support, the supporthaving two or more parallel threads, the aforesaid heating andthermo-sensitive elements comprising wires of different sizes lying indifferent threads.

In a more specific aspect the temperature responsive device isassociated with a heating circuit carrying lowvoltage, high-amperagealternating current and another circuit carrying alternating orpreferably direct current of higher voltage and lower current thanaforesaid, said heating and thermo-sensitive elements being connected insaid circuits respectively, allowing the heating element to be operatedat optimum power and the sensitive element at maximum sensitivity.

In another aspect the invention involves apparatus comprising tworesistance units, each unit including a thermo-sensitive element and aheating element, the thermo-sensitive element of each unit being inheat-transfer relation to the heating element, one unit having greatercatalytic activity on flammable gas so that the resistance of itsthermo-sensitive element is affected by heat more than that of thethermo-sensitive element of the other unit, together with an indicatorcircuit, an indicator interconnected with the thermo-sensitive elementsthrough said circuit to indicate change in the ratio of the resistancesof the elements in response to combustion of flammable gas, and acircuit independent of said circuit for energizing each of said heatingelements, whereby the heating elements may be operated at optimum powerwhile the thermo-sensitive elements are operated at maximum sensitivity.The catalytic activity of one unit may be made greater than that of theother either by forming the heating elements of materials which differin their catalytic activity or by applying more catalytic material toone unit than the other, making the two units substantially identical inall physical and electrical characteristics except in respect to theamount of catalytic material, or partly one way and partly the otherway.

For the purpose of illustration typical embodiments of the invention areshown in the attached drawing in Which- Fig. 1 is a schematic diagram ofgas analyzing apparatus employing the present temperature responsivedevice; and

Figs. 2 and 3 are side elevations, partly in section, showing variousembodiments of the device.

The gas analyzing apparatus chosen for the purpose of illustration inFig. 1 comprises a Wheatstone bridge with terminals 11 and 12 forconnection to a source of current, terminal 13 to which a galvanometer Gis connected, and resistances 6, 8, 9 and 10 in the four legs of thebridge. Resistances 9 and 10 are fixed resistances while resistance 5 isa potentiometer for balancing the bridge. The resistance units 2 and 3each comprises a supporting core 4 on which is mounted a heating element5 and a thermo-sensitive element 6. The supporting core may be formed ofany suitable porous ceramic, electrically-insulating material which isresistant to heat deterioration and is heat conductive, or may be formedof a braided asbestos tube having like properties. As indicated bybroken lines 1, the two units 2 and 3 may be located in a flow chamberhaving its inlet connected to a source of gas to be tested or analyzed.For fire detection purposes the two units 2 and 3 may be locateddirectly in the room or compartment to be protected so that the devicewill respond to unoxidized or partial products of combustion of thesmoke as soon as a fire starts. Whether or not the two units areenclosed in a flow chamber they should be close together so as to besubjected to like conditions of surrounding temperature, draft, etc.

The potentiometer 8 is set so that the galvanometer reads zero whenunits 2 and 3 are exposed to gas having no flammable component. Currentflowing through the heating element heats the elements 5 and thesupports 4 to the point where the flammable component of the gas isoxidized as it passes the heated wire. By making the catalytic activityof one of the units 2 and 3 greater that that of the other, thetemperature of its heating element 5 and the resistance of itsthermo-sensitive element 6 is raised more than that of the other unit bythe oxidation of contacting gas. Consequently the bridge is unbalancedand the galvanometer G indicates the proportion of flammable material inthe gas. Instead of the galvanometer the device may of course comprise asignal or relay for actuating any chosen protective apparatus.

In the drawings the catalytic activity of unit 3 is made greater thanthat of unit 2 by a deposit of catalytic material on the core asindicated at 7, 7a and '70. Unit 2 may also have a catalytic deposit,but if the two units are otherwise identical the deposit should be muchless than that on unit 3. While any suitable catalyst may be used,platinum or platinum-rhodium is recommended. Instead of applyingdifferent amounts of catalyst to the two units, or in addition to thisdifference, the heating element 5 of unit 3 may be formed of materialhaving greater catalytic activity than that of the heater element ofunit 2. For example, the heater element of units 2 and 3 may be formedof nickel-chromium alloy wire and platinum-iridium wire, respectively.Another way of obtaining a difference in activity between the two unitsis to make the heating elements of different sizes, as, for example,with wire or different diameters. The different ways may be employedindividually or in any combination of two or more.

By using different materials or wires of different diameters for the twoheating elements, instead of a difference in catalytic coating on thetwo units, it is easier to obtain uniformity in production because it isdifficult to gauge the amount of catalytic coating with precision. Bymaking the compensating unit 2 with little or no catalytic deposit andits heating element of material having low catalytic activity, whilemaking the heating element of the active unit 3 of material having highcatalytic activity, either with or without a deposit of catalyst, highsensitivity can be obtained because of wide difference between theresponsiveness of the two units.

According to the present invention the heating elements 5 and thethermo-sensitive elements 6 comprise separate resistor elements. Theheating elements 5 have their own terminals 14 to 17, inclusive, and maytherefore be connected in a heating circuit including an alternatingcurrent transformer 18. The thermo-sensitive elements 6 have terminals11 to 13, inclusive, which may be con- 1 nected in a direct currentcircuit including a rectifying device 19 for rectifying alternatingcurrent supplied from an alternating current transformer 20.

Accordingly, the transformer 18 may furnish to the heating element 5high-amperage, low-voltage, alternating current which affords optimumwattage with low power loss since the transformer 18 operates onalternating current with high efficiency. The thermo-sensitive elements6, on the other hand, may be operated at relatively higher voltage andlower current supplied by the rectifying device 19, thus affording agreater electrical voltage unbalance as the result of temperature changeand at the same time minimizing the cost of alternating currentrectification equipment which depends on the amount of current.

As shown in Fig 2 the insulating core 4a comprises a tube. Deposited onthe outside of the tube is a coating of oxidizing catalyst 711, althoughit should be understood the deposit may be in the form of an impregnatedlayer at the surface of the porous tube. The heating wire 5a havingterminals 14 and 15 is wound around the outside of the tube. The heatingwire may be selected without regard to its value as a temperatureindicator, and may consist of a metal alloy such as Nichrome orplatinumiridium in proportion of 80 to 20. The platinum-iridium alloy isparticularly well suited because it has a resitivity approximately threetimes as much as pure platinum so that its cross section may be threetimes as great for the same heating effect, and thus a more rugged andserviceable heating wire is available. The thermo-sensitive element 6amay be a semi-conductor such as a sintered mixture of the oxides ofmanganese and nickel, or a fine platinum wire too delicate for heaterwire but highly satisfactory as a temperature indicator. Thesemi-conductor element may be enclosed in it own protective case fittingwithin the tube 4a, or a delicate platinum wire may be sealed within thetubular core.

In the embodiment as shown in Fig. 3 the supporting core 40 comprises acylindrical body provided with two threads 21 and 22 disposed inparallel along the surface of the cylinder, the heating wire being woundin one of the threads, and the thermo-sensitive resistance 6c beingwound in the adjacent thread and the two wires being separated by theapex between the two threads. The deposit of oxidizing catalyst may beapplied as in Fig. 2.

It can be seen that because of the electrical and physical separation ofthe heating and sensitive resistances these resistances, respectively,may be selected from the most favorable materials for heating and fortemperature indicating, respectively. The heating wire preferably, butnot necessarily being exposed on the outside of the core, may be arelatively thick and durable wire, while the thermo-sensitive resistanceis preferably protected within the supporting core.

I claim:

In apparatus for detecting flammable gas, a temperature responsivedevice comprising an electrical heating resistance for raising the gascombustion temperature, a thermo-sensitive resistance responsive tocombustion of flammable gas, electrically-insulating means for holdingsaid resistances in spaced relation including a multithreaded support,said resistances comprising wires differently constructed to havedifferent catalystic activity and lying in different threadsrespectively, means for connecting said resistances in differentcircuits, and catalytic material for catalytically inducing combustionof the gas, the catalytic material being disposed on the threadedsurface of the core.

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